The present invention relates to a device for exerting an aerodynamic drag force particularly, though not exclusively, on a ballistic shell whilst in flight.
It is advantageous to be able to improve the accuracy of ballistic shells fired from artillery pieces, for example, so that there is greater probability of hitting the intended target and lower probability of so-called collateral damage. The accuracy of such shells is much greater in the azimuth direction than in the longitudinal direction. Thus, an error zone of generally elliptical shape results where the long axis of the ellipse is in the longitudinal direction.
It is possible to alter the range of an artillery shell in flight by increasing its drag coefficient.
There has been a proposal for an artillery shell which has a course correction applied to it during flight. The shell is initially aimed to overshoot the target in the longitudinal direction and, whilst in flight, applying an aerodynamic brake to cause it to fall short of the original overshoot and much closer to the target than would otherwise have been the case. In this way it is thought that an error zone of significantly smaller area may be achieved.
However, there are problems associated with applying drag increasing brakes in that the brake must be applied as symmetrically as possible about the projectile axis so as to minimise the possibility of the spinning projectile becoming unstable in its trajectory.
The present invention seeks to make possible the provision of a device able to exert a substantially symmetrical drag force about the axis of spin of a projectile so as to increase its drag coefficient during flight.
According to a first aspect of the present invention there is provided a braking device for increasing the drag coefficient of an associated projectile whilst in flight, the device comprising: at least two braking vane means which, when released, extend substantially symmetrically into a surrounding airstream whilst said projectile is in flight; retaining means for maintaining said at least two vane means in a retracted first position out of said airstream during an initial portion of said flight; releasing means to allow said at least two vanes to extend to a second position into said airstream at a desired point during said flight; and, said at least two vane means further including co-operating means to ensure substantially symmetrical deployment into said airstream.
The braking vane means may be extensible by centrifugal force due to rotation of the associated projectile about its axis.
The device is preferably positioned on the nose of the projectile, which may be an artillery shell. Shells sometimes achieve supersonic speed in flight and positioning the device on the nose of the shell ensures that the braking vane means can extend into the surrounding airstream per se.
The device may be incorporated in a fuzing device positioned on a forward part of the shell and which fuzing device arms the shell and causes it to function when required.
The braking vane means may comprise braking vane members which extend substantially normal to the projectile axis into the surrounding airstream. The braking vane members may be pivoted about an inner end such that the centrifugal force generated by the projectile spinning about its axis causes the braking vane members to extend into the airstream.
Pivoted braking vane members are advantageous over vane members which are, for example, arranged to slide out into the airstream in guide members under the action of centrifugal force. Such sliding vanes have limited area available to extend into the airsteam due to the need to maintain adequate support of the vanes within the device to counteract the stresses imposed on them by the airstream. Furthermore, unless such sliding systems are very accurately made, they have a tendency to jam due to any misalignment which may be present. Thus, such sliding systems are inherently more expensive to make and less efficient in operation.
Pivoted vane members are advantageous under spin conditions because the distance between the pivot point and the centre of gravity of the vane members provides the mechanical advantage of allowing the pivoted vane members to deploy under less force than said sliding vanes, due to the turning moment generated during deployment. Pivot vane members also have the advantage of not requiring guide members, and so the misalignment of vane members and their guide members does not create a problem.
The retaining means may be a cover member which surrounds the braking vane members during an initial part of the flight so as to prevent them extending until desired.
The retaining means may be one or more straps.
The retaining means may be latches or hooks positioned on a support or base member in a way which prevents the braking vane members extending until desired.
The retaining means may be one or more pins which may extend into or through at least one braking vane member and a support or base member.
The releasing means may be explosive releasing means such as a small explosive charge or explosive cord for example, or may comprise a gas motor device. The releasing means may be detonated, for example, by a remote radio signal at the appropriate time so as to cause the retaining means to release the braking vane means to deploy by extending out into the airstream. The releasing means may cause fracture of the retaining means. The releasing means may alternatively cause the retaining means to move to a position which allows the braking vane means to deploy.
The releasing means may achieve its object by causing a retaining cover member to fracture and/or be jettisoned from the shell.
The releasing means may alternatively cause frangible fingers which interlock the braking vane means together to break and allow them to deploy through slots, for example, in a nose cover member.
The releasing means must be actuated at the appropriate time in order to provide the desired course correction. The releasing means may be activated as stated above by a remote radio signal. The device of the present invention itself may comprise a radio receiver device to receive the remote radio signal and to cause activation of the releasing means. Alternatively, any such radio receiver device may be associated with a fuzing device or with the shell itself, the radio receiver merely being operatively connected to the releasing means. The remote radio signal may come from a ground control station or a reconnaissance aircraft, for example.
Alternatively, the releasing means may be actuated by use of the Global Positioning System (GPS) as follows. At a given. point in its trajectory, an on-board processor compares the predicted position of the projectile with its actual position as determined through remotely accessing the GPS. The processor then calculates the appropriate time delay at which the braking vane means need to be deployed, in order to provide the proper course correction, to bring the projectile on course for its intended target. The processor then sets an on-board timer accordingly, and the timer actuates the releasing means after the said appropriate time delay.
The braking vane means also employs co-operating means to ensure that, in use, they deploy substantially symmetrically about the axis of the shell. Such means may comprise control areas of the braking vane members, the control areas being arranged such that any asymmetric extension of radially adjacent vane members would result in mechanical interference between the control area of one vane member and an adjacent part of the other vane member. Thus, if one vane were to jam or stick in the closed or partially extended position, the control area of the adjacent vane would prevent the adjacent vane from extending further and substantially preventing asymmetrical deployment from occurring.
Alternatively, intermeshing gear teeth may be employed on curved portions of the braking vane members which ensure that they are deployed symmetrically.
The device may comprise pairs of braking vane members, each pair being disposed axially adjacent another.
The device may include means for preventing the braking vane members from extending further than desired into the airstream.
The device may comprise twin-bladed pairs of braking vane members where the twin blades are axially adjacent each other. Both blades may be pivoted about an inner end such that the centrifugal force generated by the associated shell spinning about its axis causes both blades of the braking vane members to extend into the airstream. One of the twin-blades may be prevented from extending as far into the airstream as the second of the twin-blades. The second of the twin-blades may be prevented from extending further than desired into the airstream by restraining means which may be carried by the first blade. The second of the twin-blades may overlie the first blade such that support is provided for the second blade by the first blade when both blades are fully extended. The second blade may advantageously provide an increased area extending into the airstream and therefore an increased drag coefficient for the shell in flight.
According to a second aspect of the present invention, there is provided a fuzing device incorporating the braking device of the first aspect of the present invention.
According to a third aspect of the present invention, there is provided a projectile incorporating the braking device of the first aspect or the fuzing device of the second aspect of the present invention.